fix: avoid executing actions when completing -- in v2

[happysnaker]

What type of PR is this?

• bug

What this PR does / why we need it:

Prevents --generate-bash-completion from falling back to normal command execution when the user is completing after -- in v2.

The change keeps the existing v2 behavior that disables completion after a positional separator that already appeared earlier, but trims the completion flag before normal dispatch so the command action is not executed accidentally.

It also preserves completing -- itself, which allows the shell script to request long-flag suggestions without running the action.

Which issue(s) this PR fixes:

Fixes #1993

Special notes for your reviewer:

I kept this intentionally minimal for v2-maint:

• no shell script changes
• no behavior change for arguments after an earlier --
• added regression coverage for both the helper and App.Run

Testing

Added regression tests for:

• checkShellCompleteFlag when -- is the token being completed
• checkShellCompleteFlag when -- already appeared before completion
• App.Run to ensure double-dash completion does not execute the action

Release Notes

Prevent v2 shell completion from executing command actions when completing after `--`.

[happysnaker]

Quick follow-up after checking the latest PR runs: the behavior change itself is covered by the new regression tests and the substantive remaining failure is the existing binary-size gate on the v2 test workflow (2.3MB vs current 2.2MB max). The separate lint failure appears to come from the runner/toolchain mismatch (file requires newer Go version go1.26 (application built with go1.24)), which matches what I saw on the previous revision as well.

If you would prefer, I can try a follow-up commit that trims this patch further or moves the guard/test shape around to see whether it avoids the binary-size bump.